1. Field of the Invention
The present invention relates to a method and apparatus for detecting leaks in fluid-treating hollow fiber membrane modules without the use of liquids. More particularly, the invention relates to an optical method and apparatus for locating leaks from hollow fiber membrane modules having a housing and one or more bundles of hollow fibers positioned within the housing.
2. Brief Description of the Prior Art
Fluid separating apparatus employing hollow fiber permeable membranes are well known. Such apparatus have heretofore been employed widely for applications such as dialysis, ultrafiltration, reverse osmosis, fractionation of gaseous mixtures and the like. The permeable membranes used in such apparatus are mostly made of cellulose or its derivatives. In addition to those conventional materials, attempts have been made to utilize a variety of other materials for the fabrication of permeable membranes. Examples of such other materials include polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, ethylene-vinyl alcohol copolymer and the like.
A large majority of such permeable membranes are of the wet type. That is, the membranes are kept wetted with an aqueous solvent from the time they are produced until the time they are put into use. If such a membrane is allowed to dry prior to being put into use, its micro-structure is irreversibly altered in a way which seriously degrades or even destroys its permeability characteristics.
Recently, dry permeable membranes, which are distinctive from the above-mentioned membranes, have been manufactured. The permeability of these dry membranes is not altered if the membrane becomes dry before it is put into use. For that reason, the dry membranes can be much more easily handled, stored and transported.
When a permeable membrane is used in a medical apparatus such as an artifical kidney, a leak in the membrane causes a loss of blood and, therefore, should be prevented by all means. For medical purposes, even a single leak per 20,000 hollow fibers in a membrane module is unacceptable. Therefore, leak tests have been performed on fluid-treating membrane modules incorporating hollow fibers. Conventional leak tests are applicable to conventional wet membranes. In such membranes, one or more bundles of hollow fibers are positioned in a case or housing with one end of each bundle being sealed by a cemented layer. A leaky fiber in such a wet membrane can be detected by submerging the cemented ends of the module into a liquid such as water. A gas such as air is applied under pressure to the external surfaces of the hollow fibers in the module. Gas passing through leaks at the cemented end of the hollow fiber bundles would be detected as bubbles in the water.
Another way of detecting leaks from conventional wet membranes is based on the fact that the gas permeability of a wet hollow fiber is low. The inlet and outlet pressures of gas applied to the module are monitored. A pressure reduction between the inlet and outlet sides of the module indicate an abnormal flow of gas due to a leak.
The above-described methods can not be used to detect leaks in dry hollow fiber membrane modules. If a dry hollow fiber is wetted and then dried, its micro-structure may be so altered as to degrade or destroy its permeability properties. Furthermore, generally speaking, a dry permeable membrane has a high gas permeability. Therefore, the static pressure reduction method described above will not produce a detectable pressure difference and can not be successfully applied to dry membranes.